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这是传给结束时候要执行的函数的，functions bound via System_atexit会调用这些函数，支持传进去一个参数，也就是BIOS_exit()中传进去的参数。
下面是手册上对该函数的解释：
VoidBIOS_exit(Intstat);
ARGUMENTS
stat— exit status to return to calling environment.
DETAILS
This function is called when a SYS/BIOS executable needs to terminate normally. This function sets the internal SYS/BIOS threadType to ThreadType_Main and then calls System_exit(stat), passing along the 'stat' argument.
All functions bound via System_atexit or the ANSI C Standard Library atexit function are then executed.
